In the last two decades, surveying instruments have undergone a substantial evolution, particularly to meet the demands of increasing accuracy and ease of use. For example, distances are now routinely measured very accurately using an electronic distance meter (EDM). An EDM transmits a pulsed light beam (generally a coherent beam of near-infrared light) to a distant target. A "retroreflector" is placed at the target to return the beam back to the EDM. A retroreflector is a device capable of causing reflected radiation to return along paths substantially parallel to the paths of the corresponding incident radiation. Some conventional EDMs measure distances by accurately measuring the elapsed time between the instant the pulsed light beam left the EDM and the instant the beam returned to the EDM after being reflected from the retroreflector, then electronically computing half the distance traversed by the beam. Since light travels through the atmosphere at a substantially constant known velocity, the distance traveled by the light beam can be readily calculated from the time measurement. Most types of EDMs measure the phase shift of the returning beam relative to the transmitted beam and compute the traversed distance from that information.
Another key evolutionary step has been the integration of a highly accurate angle-measuring device (theodolite) with an EDM to form what is known in the art as a "total station."
Before the advent of the total station, EDMs were generally used either alone or mounted piggyback on top of a theodolite. The piggyback configuration was usually used with a retroreflector and separate theodolite target that had to be carefully spaced apart to avoid possible measurement discrepancies due to convergence. Total stations, in contrast, are coaxial in that the theodolite and EDM "axes" are coincident. With a total station, the center of the retroreflector serves as the reference point for all angle and distance measurements, thus eliminating the need for a separate theodolite target.
A preferred retroreflector for use with an EDM or with a total station is a "corner-cube prism." A conventional corner-cube prism is made from a solid cylindrical piece of optical glass. The prism has a "face": a circular planar surface, normally facing the EDM, that is perpendicular to the longitudinal axis ("optical axis") of the prism. The optical axis passes through the center of the face. The rear of the prism is configured as a pyramid having three facets perpendicular to each other and equiangular to the optical axis so as to form an apex on the optical axis opposite the face. Since the three facets, relative to the face of the prism, are situated at angles greater than the "critical angle," an incident light beam entering the prism through the face is internally reflected off each of the prism sides so as to exit the prism through the face.
A corner-cube prism is typically encased in a protective enclosure (or "canister"). The canister is mounted on a tripod or surveyor's prism pole used to hold the prism stationary during use. The tripod or pole is typically configured to allow the prism elevation to be adjusted and the prism to be leveled. The axis of the pole or tripod, referred to herein as a "target axis," is typically oriented vertically so as to enter the earth at the "target point" (i.e., the point having a distance from the EDM that is to be measured).
The accuracy of distance measurements performed using an EDM and retroreflector is dependent mainly upon the electronic accuracy of the EDM and on the position of the retroreflector relative to the target axis. A light beam, transmitted from an EDM to a retroreflector, requires a certain amount of time to travel the distance from the EDM to the retroreflector and back again. For "elapsed time" EDMs, that time increment should represent very accurately the actual distance from the EDM to the target axis. Similarly, the magnitude of the phase shift experienced by a light beam traveling from a "phase shift" EDM to a retroreflector and back again should also represent very accurately the actual distance from the EDM to the target axis. However, because the prism refracts light (i.e., light travels more slowly through the prism glass than through air), a discrepancy can be imparted to the distance calculation displayed by the EDM. This discrepancy can be significant, particularly if the prism were mounted on the tripod or pole with the prism apex situated on the target axis (which some persons skilled in the art regard as the "correct" position for the prism). The slower velocity of light in glass, relative to the velocity of light in air, is quantitatively indicated by the index of refraction of the prism glass. For example, with typical prisms having an index of refraction of 1.509, the distance traveled by the light beam inside the prism multiplied by 1.509 is equal to the distance the beam would have traveled in the same amount of time in air. The difference of these two distances is the amount of discrepancy that would result in a distance measurement if the apex of the prism were placed on the target axis. Manufacturers of conventional corner-cube prisms attempt to compensate for this error by mounting the prism a distance away from the target axis toward the EDM equal to the computed distance difference. This displacement of the prism relative to the target axis is conventionally termed the "zero offset" position.
Ideally, a retroreflector should produce a reflected light beam that is exactly parallel to the incident beam, even when the incident beam is not exactly parallel to the optical axis of the retroreflector. This is particularly desirable whenever the retroreflector is located a long distance from the EDM. Corner-cube prisms inherently have the capability of producing a parallel reflected beam. But, when a corner-cube prism is adjusted relative to the target axis to have "zero offset", it virtually never produces a reflected beam that is exactly parallel to the incident beam except when the incident beam is parallel to the optical axis of the prism. As a result, extremely accurate alignment of the optical axis of the prism with the propagation vector of the incident beam is required to ensure that the reflected beam returns to the EDM. Performing this alignment under field-use conditions is very difficult and time-consuming.
U.S. Pat. No. 3,748,026 to Scholdstrom discloses equations that define, for a particular corner-cube prism, the position of a "pivot axis" intersecting the optical axis of the prism between the base and apex of the prism. The equations are derived from certain relationships purportedly applicable for minimizing distance measurement discrepancies that would arise if the optical axis of the prism were excessively misaligned relative to the incident light beam. According to Scholdstrom, the pivot axis actually represents one end of the distance to be measured; i.e., the prism should be positioned on a mounting such that the pivot axis crosses the target point. However, this reference does not disclose how the pivot axis for a particular prism can be determined other than mathematically. Of course, the results of such calculations will be different for different prisms. Also, such calculations cannot practicably be performed in the field.
Different prism manufacturers apply different amounts of offset to their prisms in accordance with conventional theory and with their respective different opinions as to what the optimal offset should be. Thus, fitting a first manufacturer's prism to a second manufacturer's prism mounting typically changes the prism's offset by an unknown amount, leading to distance measurement discrepancies. This is a problem in the field because most surveyors use equipment made by more than one manufacturer. Some artisans have resorted to individually measuring each of their prisms to determine the offset of each. Unfortunately, this is a costly procedure and is often impossible because many prisms are hermetically sealed in their housings. Even with prisms made by the same manufacturer, accumulated manufacturing tolerances in prism housings, mountings, and other mechanical components can introduce significant offset discrepancies from prism to prism and from mounting to mounting.
Hence, there is a need for methods and apparatuses for positioning a corner-cube prism or other retroreflector relative to the target axis so as to allow the prism to produce a reflected beam that is parallel to an incident beam, even when the incident beam is not parallel to the optical axis of the prism.
There is also a need for such methods and apparatuses that are easily and conveniently usable in the field.
There is also a need for retroreflector apparatuses that can be set up at a location remote from a source of electromagnetic radiation, such as an EDM, and that will produce a reflected beam from an incident beam, transmitted to the prism from the source, that is exactly parallel to the incident beam, even when the face of the prism is not exactly perpendicular to the incident beam.
There is also a need for prism-mounting apparatuses that allow prisms from various manufacturers and of different diameters to be interchangeably used without generating discrepancies in distance measurements.